The invention relates to mirror assemblies for scanning incoming radiation from a scene onto one or more radiation detectors. The invention also relates to imaging and other systems using such mirror assembles.
In many known thermal imaging systems, radiation from a scene to be imaged is scanned onto either a single detector or a detector array by means of one or more scanners. One known scanner is a polygonal scan mirror.
A known polygonal scan mirror is shown in FIG. 1, which schematically shows a cross-section through the scan mirror perpendicular to the axis 10 thereof. The scan mirror of FIG. 1 has six mirror faces 12. Each mirror face 12 has a planar surface so that all light which is incident at a given angle of incidence is reflected in a single angle of reflection (for a given position of the scan mirror). Moreover, each mirror face 12 subtends an angle .alpha. with respect to axis 10.
In the operation of the known scan mirror, radiation 14 which is incident on a mirror face 12 is reflected onto a detector, a detector array, or some other target (not shown in FIG. 1). As the scan mirror is rotated around axis 10, successive bundles of parallel rays (each bundle having a different angular orientation) are reflected onto the target. After the scan mirror rotates through an angle of .alpha., the next mirror face 12 functions to reflect the radiation bundles onto the target. In this way, the target is scanned six times for each complete rotation of the scan mirror of FIG. 1.
A significant problem with the known scanners of FIG. 1 is that they generally are operated at very high rates of rotation, for example (39,375 or 78,750) revolutions per minute. This high speed operation generates high windage losses and creates severe acoustical noise unless the mirror is operated in a partially or completely evacuated chamber. Such operation is difficult and expensive to implement.